This question is a bit difficult. First of all, I want to ask how your pitch is? If your pitch is okay, you can find an electronic keyboard or piano. Slowly let him sing the scales with you (do, re, mi, fa, so, la, xi, do). If everything goes well, the intonation problem can be solved in a month. Also, don’t let him wear earplugs while listening to music and singing. This is the biggest factor leading to acquired tone deafness. .Try to separate singing and listening.

It is best to find a student or teacher from a conservatory of music to teach

The above is my personal experience. I only studied 6 years in a conservatory. Singing in the New Year. Please forgive me if I am wrong. Please give me more suggestions...